monitor, keyboard and mouse not responding

I am trying to fix my mother-in-laws Dell Dimension 3100/E310. I'm not a guru but I have really good understanding of these crazy machines and have fixed many of my own problems. but I can't seem to figure this out.\r\n
She said it was working fine before she moved. it has been unplugged for about 2 months then they try to use it and the monitor stays in sleep mode. The mouse or keyboard will not even respond... almost like nothing is getting any power from the computer. When the unit is on, the fan will run but doesn't sound like it is trying to "work". I tried a cd with no luck.

I'm thinking it may be the motherboard but another blog suggested the graphics card. would that cause both problems with monitor and keyboard and mouse?

Wild guess on my part would be the power supply unit - but that's just a guess.
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The 3100 has some diagnostic lights on it. Here's a list (scroll down to System Lights) that might help you find the problem. Any beeps when you power it on? Beep codes can help too. Also look at the Diagnostic Lights section.Documentation

What computer do you have? And please don't say a white one. - Sheldon Cooper